1. Field of the Invention
The present invention relates to a power supply circuit which supplies power from a primary side to a secondary side under the control of a microcomputer and prevents the supply of power from the primary side to the secondary side at the time of occurrence of an abnormality and a flat-screen television set employing the power supply circuit.
2. Description of the Related Art
FIG. 8 is a block diagram showing the configuration of a conventionally known power supply circuit. A power supply circuit 1 generates steady power from primary AC power such as commercial power and supplies it to a secondary side. The power supply circuit 1 includes a rectifier circuit 2 for converting AC voltage to DC voltage, a smoothing circuit 3 for smoothing the converted DC voltage, a switching circuit 4 for converting the smoothed DC voltage to AC voltage again, a main transformer 5 for supplying power while isolating the AC voltage converted by the switching circuit 4 from a secondary side, and secondary power supply circuits 6 which are supplied with predetermined voltages generated by dividing the output voltage of the main transformer 5. Further, according to a microcomputer 7 in a secondary circuit, the power supply circuit 1 performs feedback control of the switching circuit 4 based on an output voltage of the secondary power supply circuits 6.
In the power supply circuit having the above-described configuration, AC power supplied from a primary power supply 200 such as a commercial power supply is rectified by the rectifier circuit 2 and smoothed by the smoothing circuit 3 into DC power. The DC power is converted to AC power again, which is supplied to the secondary power supply circuits 6 isolated by the main transformer 5.
The switching circuit 4 converts input power from DC to AC, under the control of the microcomputer 7 using an activation signal outputted to the switching circuit 4. The switching circuit 4 includes a logic IC 4a which performs PWM control, and the activation signal outputted from the microcomputer 7 is inputted to an input terminal of the logic IC 4a. The activation signal activates the logic IC 4a, which converts power from DC to AC. Further, power outputted through the main transformer 5 is supplied to a plurality of secondary power supply circuits 6a, 6b, and 6c, which boost or reduce input DC power voltage to supply power to circuits.
In the power supply circuit 1, the microcomputer 7 controls the activation of the switching circuit 4. When the microcomputer 7 detects an abnormality in the power supply circuit, the microcomputer 7 stops the activation of the switching circuit 4 to prevent the supply of power to the secondary power supply circuits 6 through the main transformer 5.
However, in the power supply circuit having the above-described configuration, in the case where the microcomputer 7 malfunctions or breaks down, it becomes impossible to prevent the supply of power from the primary side to the secondary side in the power supply circuit 1. The stop of the activation of the microcomputer 7 can be caused by a latch-up or a breakage. In either case, the microcomputer 7 cannot control the switching circuit 4.
On the other hand, there is disclosed a driving control unit for detecting an abnormality of a microcomputer and driving a load even at the time of stop of the microcomputer due to a breakage or the like. The driving control unit includes a reset IC with a watchdog function for outputting a reset pulse at the time of detecting an abnormality of the microcomputer, a driving signal output unit for, upon receiving the reset pulse, integrating a power supply voltage and outputting a driving voltage, and switching means for driving a load using this driving signal.
In the driving control unit, when the microcomputer stops its function, the reset IC with the watchdog function outputs a reset pulse to the driving signal output unit. Upon receiving the reset pulse, the driving signal output unit accumulates a driving signal and outputs it to the switching means, which drives the load (e.g., see Japanese Patent Application Laid-Open No. 2002-268909, which is hereinafter referred to as Patent Document 1).
There is also disclosed an abnormality detection apparatus for detecting a microcomputer that is operating abnormally among a plurality of microcomputers included in a control apparatus.
In the abnormality detection apparatus, one of the microcomputers that has received an abnormality detecting signal transfers the signal to the other microcomputers, and microcomputers that have received the transferred signal send it back to the microcomputer that has sent the signal, thereby detecting an abnormal microcomputer which has not sent it back within a predetermined time (e.g., see Japanese Patent Application Laid-Open No. 2002-41329, which is hereinafter referred to as Patent Document 2).
There is disclosed a method for detecting an abnormal operation of a microcomputer without providing a particular monitoring program or an input/output terminal in a similar control apparatus including a plurality of microcomputers (e.g., see Japanese Patent Application Laid-Open No. 10-127061, which is hereinafter referred to as Patent Document 3).
Further, there is disclosed a power control apparatus for cutting off power based on an abnormality occurrence in a secondary circuit or an instruction from a microcomputer.
In the power control apparatus, a switch is interposed between a primary power supply and a secondary power supply, and a power-off signal is outputted from the secondary power supply in response to an abnormality in the secondary power supply. The power-off signal actuates the switch to cut off power supplied from the primary power supply to the secondary power supply (e.g., see Japanese Patent Application Laid-Open No. 2001-136661, which is hereinafter referred to as Patent Document 4).
There has been the following problem in the invention described in Patent Document 1.
In the case where the IC with the watchdog timer breaks down, it is impossible to perform abnormality processing on the power supply circuit. The IC with the watchdog timer may break down due to a latch-up caused by noise as in the case of the microcomputer. Therefore, in the case where the IC with the watchdog timer breaks down, it becomes impossible to cut off the supply of power from the primary side to the secondary side as in the case of the microcomputer. Further, the invention described in Patent Document 1 drives the load at the time of a failure of the microcomputer, whereas the present invention prevents the supply of power from the primary side to the secondary side to stop the load; therefore, these inventions differ in the objects.
In the inventions described in Patent Documents 2 and 3 which include a plurality of microcomputers, in the case where all the microcomputers break down, it becomes impossible to solve the problem as in the case of the invention described in Patent Document 1. Further, the inventions described in Patent Documents 2 and 3 are based on a plurality of microcomputers, and there is not disclosed a method for stopping the load at the time of detection of an abnormality of a microcomputer as in the case of the present invention.
Further, there has been the following problem in the invention described in Patent Document 4.
The invention described in Patent Document 4 provides a cutoff switch controlled by the microcomputer between the primary side and the secondary side. Therefore, in the case where the microcomputer breaks down, it is impossible to control the switch so that the problem of the present invention cannot be solved.